With the progress of electronic photograph type image forming apparatuses, image forming apparatuses with higher performance are needed, and image forming handling five or more colors instead of conventional image forming using four colors has been proposed. There are various methods, such as a method that adds special colors, i.e., red, blue and green, or gold, silver and fluorescence colors, to the standard four colors: cyan (C), magenta (M), yellow (Y) and black (K), or a method that adds light cyan (Lc) and light magenta (Lm), which is general in inkjet methods. However, the purpose is to differentiate image characteristics to be generated.
There are various types in image forming apparatuses handling more colors. For example, in the case of image forming apparatuses using developing agents (toner or powder ink) of six colors, there are generally an image forming apparatus using six image holders (photoconductors) that corresponds to each color toner shown in FIG. 1 and an image forming apparatus using one photoconductor shown in FIG. 2. In addition, there is also an image forming apparatus using two photoconductors, which is described in Japanese Patent Laid-Open No. 4-204871, shown in FIG. 3.
In the apparatus shown in FIG. 1, image forming parts Sa-Sf, in which six photoconductors 1a-1f and developing devices 41-46 filled with developing agents having different spectral characteristics are made to correspond respectively, are placed along the conveyance path of a middle transfer belt 5. This method is a productivity-oriented approach in which degradation of an image output rate is suppressed as much as possible for four-color image formation.
On the other hand, the apparatus shown in FIG. 2 has a structure, in which a photoconductor 1 is combined with six developing devices 41-46, and an arbitrary developing device is selectively moved to a position opposite the photoconductor 1 by rotating a rotary 4 mounted with the developing devices 41-46, to sequentially develop latent images. Therefore, a toner image is transferred on the middle transfer belt 5 for every color to perform multiplex transfer of toner images of six colors, and then the toner images of six-colors formed on the middle transfer belt 5 are transferred on a recording paper. According to this method, an image formed with six-color toner can be outputted while still suppressing the volume of an apparatus to the minimum.
In addition, the apparatus shown in FIG. 3 is a compromise between the apparatuses shown in FIGS. 1 and 2. That is, the apparatus has two image forming parts: an image forming part Sa that includes a photosensitive drum 1a and a second image forming part Sb that includes a photosensitive drum 1b. The image forming parts Sa and Sb have three developing devices 41-43 or 44-46, respectively.
As will be described below in detail, a structure of the control unit of an apparatus that uses color components of five or more colors is almost the same as that that uses color components of four colors in the range of RGB data. However, above a structure, in which RGB data is color-converted into image forming colors (i.e., YMCK, and Lc and Lm), a five-or-more-colors-capable structure is required to handle five or more colors.
In such a structure, especially, a color converter is required to perform high speed processing, so that it is may often be implemented as hardware. However, the scale of hardware for a five-or-more-colors-capable color converter may be large, so that the cost rises. In addition, a four-colors-capable color converter is easily available, so that it is desirable to support five or more colors effectively using the four-color color converter.